Electro-optical storage device

ABSTRACT

An electro-optical material body has two opposed surfaces, each covered with a transparent photoconductive material layer covered, in turn, with a transparent conductive electrode. Although not necessary, in a preferred method of operation, information is written into the body using light which is directed onto the body surfaces at an angle of inclination which is the same as that used with the light used to read information from the device. In a different embodiment, one surface of the body is covered by a transparent photoconductive material layer covered, in turn, by a transparent electrode, while the other surface of the body has a plurality of spaced electrodes thereon.

I 1!" if 5 1 0 llniteel Sta 1111 3,747,075 Keneman et al. July 17, 1973ELECTRO-OPTICAL STGRAGE DEVICE OTHER PUBLICATIONS [75] Inventors: ScottAllen Keneman, Hi ht t Bell System Technical Journal, Image Storage &Dis- George William Taylor, Princeton, play Devices Using Fine-Grain,Ferroelectric Ceramboth of N,J ics by Meitzler et al., July-Aug. 1970,pages 953-967. K [73] Assignee: RCA Corporation, New York, N.Y.

Primary ExaminerStanley M. Urynowicz, Jr. [22] Filed: Apr. 3, 1970Att0rneyGlenn H. Bruestle, M. Epstein et al.

[21] Appl. No.: 25,397

[57] ABSTRACT [52] Us. CL 340/173 LT 340/1732 340/173 LM Anelectro-optical material body has two opposed sur- 250/219 faces, eachcovered with a transparent photoconduc- 151 int. c1 01 1c 11/22 materiallayer Wered, will a transparent [58] Field of Search 340/173 2 173 LMconductive electrode. Although not necessary, in a pre- I ferred methodof operation information is written into 340 173 LT, 173 LS, 350150, 250219 l Q the body usmg light which 1s d1rected onto the body [56]Rehremes cued surfaces at an angle of inclination which is the same asthat used with the light used to read information from UNITED STATESPATENTS the device. In a different embodiment, one surface of 3,609,0029/l97l Fraser et al 340/1732 h body is covered by a transparentphotoconductive 3'374473 3/1968 cummms'ff 340/1732 material layercovered, in turn, by a transparent elec- 3643'233 2/1972 Fan et a]340/1731 trode, while the other surface of the body has a plural-3,3.l9,235 5/1967 Chang et a]. 340/174 YC i y of spaced electrodesthereon FOREIGN PATENTS OR APPLICATIONS 6 C: i 6 D i i 873,897 8/1961Great Britain 340/1731 a gums a 4t: 265 f6 /1'// l M 1 :11 4 48 4 M 1' Leat r milk m mwumews ATTORNEY ELECTED-OPTICAL STORAGE DEVICE BACKGROUNDOF THE INVENTION The invention herein disclosed was made in the courseof or under a contract or subcontract thereunder with the Department ofthe Air Force.

This invention relates to optical storage devices comprisingelectro-optical materials. By electro-optical is meant crystallinematerials containing electrically polarizable domains having opticalproperties.

In such devices, optical information is stored as one of two remanentpolarization states between which individual elemental regions ordomains of an electrooptical material, e.g., a ferroelectric material,can be switched by the momentary application of an electric field, andis read from the device using optical means. In one form of suchdevices, a ferroelectric material body has two major, opposed parallelsurfaces. One of the surfaces is covered with a photoconductivematerial, covered, in turn, by a first transparent electrode, and theother surface is covered by a second transparent electrode.

Write-in of information is accomplished by applying a voltage betweenthe two electrodes and directing a pattern of light onto thephotoconductive material side of the body. The light, where it falls onthe photoconductive material, reduces the otherwise high electricalresistivity of the photoconductive material. This results in theapplication of an electric field through the ferroelectric body in apattern corresponding to the pattern of the incident light, and anattendant polarization to one of the two remanent states of thoseferroelectric domains which are within the electrc field.

Read-out of information is accomplished by shining light through theferroelectric material through crossed polarizers. By proper selectionof the polarizer axes relative to the domain axes, light will eitherpass through or be blocked by the combination of polarizers andferroelectric material depending upon the remanent polarization state ofthe domains of the ferroelectric material through which the lightpasses.

Owing to such factors as fringing electric fields within theferroelectric body, and the need for utilizing angled light rays in theread-out of information, such factors being described hereinafter, aproblem with the prior art devices is that of obtaining good opticalresolution, i.e., high density storage and read-out of opticalinformation.

DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 are cross-sectional viewsillustrating details of and the operation of prior art devices; and

FIGS. 3 through 6 are cross-sectional views illustrating details of andthe operation of devices according to the instant invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The basic principles ofoperation, and examples of one type of prior art electro-optical storagedevice using a ferroelectric material, are provided in U.S. Pat. No.3,374,473, issued to S. E. Cummins on Mar. 19, 1968.

With reference to FIG. 1 herein, the ferroelectric material portion of aprior art information storage system is shown. The ferroelectricmaterial 10, which can be.

single crystal bismuth titanate (Bi Ti O as a preferred material, is inthe form of a flat plate having two major, opposed parallel surfaces 32and 14. Other known ferroelectric materials, such as single crystaltrigonal boracites, single crystal gadolinium molybdate or fine orcoarse grained lead-zirconate-titanate (PZT) ceramics, can be used.Covering the surface 12 is a layer 16 of a light transmitting (e.g., 50percent light transparent) photoconductive material, e.g., zinc selenideor cadmium sulfide, covered, in turn, by a layer l8 of a transparent,electrically conductive material, e.g., tin oxide. Covering the othersurface 14 of the crystal 10 is a transparent, electrically conductivelayer 2% similar to the conductive layer 18. The two layers l3 and 20comprise electrodes for the crystal ll) across which a voltage isapplied in use of the device.

The applied voltage is of such magnitude as to be capable, when appliedacross the crystal It of switching elemental areas or domains of thecrystal between two polarization states, between the polarity of theapplied voltage. Owing to the presence of the photoconductive layer 16,of high electrical resistance when not illuminated, the applied voltageis insulated from the crystal 10, causing no polarization reversal ofthe domains thereof, until low resistance paths through the layer 16 arecreated by the illumination of the layer 16. This is describedhereinafter.

The crystallographical orientation of the crystal 10 is selected, in amanner disclosed in the aforementioned patent,'such that the applicationof an electric field of one polarity through the crystal 10, via theelectrodes 18 and 20, results in a polarization of the crystal domainswithin the electric field, i.e., a shifting of the opticalcharacteristics of the domains within the field to a particular one oftwo stable optical states. Reversing the electric field causes thedomains in the field to be polarized to the other of the optical states.By domain is meantthe smallest region of the crystal which can bealternately polarized, independently of adjoining domains, by theapplication of an electric field thereto. Once set in one of the twopolarized states, the crystal domains remain in that state upon removalof the electric field. Also, the polarizing effect is voltagetimedependent, i.e., the shorter the period the electric field is appliedacross the crystal, the greater the strength of the field must be tocause polarization of the domains.

In use of the device, information is written into the device byprojecting a pattern of light onto the electrode 18 side of the crystal10. For convenience of illustration and description, the writing lightis shown as a small diameter, circular beam 24, i.e., a pencil" lightbeam. The angle of incidence of the writing beam 24 with the surface 12of the crystal it) is immaterial. Also, the bending of the beam 24 atthe various material interfaces is not shown. V

Where the light beam 24 passes through the photo conductive layer l6 theelectrical resistance thereof is significantly reduced, whereby thevoltage on the electrode I8 is applied to the crystal surface 12 at anarea 26 intersected by the light beam 24. That is, the area 26, definedby the light beam 24, acts as an elemental electrode on the surface 12of the crystal 10, whereby an electric field is established through thecrystal ll between the elemental electrode 26 and the electrode 20 onthe other surface I4 of the crystal 10.

The electric field associated with one elemental electrode 26a isindicated by the use ofdash lines 30 represcnting some of the electricfield lines (the writing beam 24 giving rise to the electrode 26a notbeing shown for greater clarity). As known, the strength of an electricficld at any point is approximately inversely re latcd to the length ofthe particular electric field line passing through the point. As shownin MG. l, the ends of the field lines terminate at the elementalelectrode 26a and diverge as they extend through the crystal toterminate at the electrode 20. The further the lateral distance of thelines from the electrode 26a, the greater is the length of the lines,indicating that the strength of the electric field decreases withlateral distance from the electrode 26a.

Depending upon the magnitude and duration of the applied voltage,various domains within those portions of the electric field ofsufficient strength to polarize the domains are polarized to one of thetwo polarization states. The group of polarized domains, adjacent to oneanother and disposed within a region of the crystal, correspond to asingle bit of stored information. Two such regions, referred tohereinafter as information bit regions, are designated by the numeral 23in FIG. 1.

A problem with the prior art devices is that while the strongestelectric field is within aprojection of the elemental electrode 26a,(the projection area being designated by the dash-dot lines 32), thestrength of the electric field outside this projection region, referredto hereinafter as the fringing electric field, falls off relativelyslowly with lateral distance from the elemental electrode 26a. That is,the rate ofincrease of the length of the field lines 30 shown in FIG. 1is ralatively small with increasing lateral distance of the lines fromthe electrode 26a. The effect of this strong fringing field is thatvarious domains, laterally spaced from the electrode 26a, are likely tobe poiarized. To the extent that the fringing electric field polarizesdomains laterally spaced from the elemental electrodes 26, therebyincreasing the width ofthe information bit regions 28, the resolution ofthe device is limited. That is, each elemental electrode, correspondingto one information bit, must be spaced at least sufficiently far fromthe other elemental electrodes 26, as shown to the right of FIG. 1, suchthat no overlapping of the information bit regions 28 occurs within theprojection areas of the electrodes 26. If such overlapping did occur,information written into the device at one point could be erased byinformation written into the device at an adjacent point.

A further limitation on the optical resolution of the prior art devicesarises from the fact that the axis of the electric field, indicated bythe arrow 34 in FIG. ll, between the elemental electrode 260 and theelectrode 20 is perpendicular to the crystal surfaces l2 and 14. Thus,the axis of each information bit region 28, indicated by the arrow 35 inFIG. l, is also perpendicular to the crystal surfaces.

As described in the aforementioned patent, information read-out isaccomplished by placing the crystal between crossed polarizers,directing light through the polarizers and crystal, and detecting thepattern oflight which emerges from the system. That is, light passingthrough crystal domains in one state ofremanent polarization is sooptically polarized as to pass through the system, while the lightpassing through the domains in the other remanent polarization state isso optically polarized as to be blocked by the system.

The further limitation on the optical resolution of the prior artdevices arises by virtue of the fact that the reading light, todistinguish between domains in different polarized states, must bedirected through the crystal at some angle, dependent upon theparticular crystal material, other than to the crystal major surfaces 12and 14. For a crystal it of bismuth titanate, for example, having threeaxes a, b, and c, as indicated to the left of FIG. 1, the angle of thereading light is preferably between l0 and 30 from the c axis, i.e., thenormal to the crystal surfaces. No inclination of the reading lightrelative to the a axis is required. The effect of the use of an inclinedreading light is described in connection with PEG. 2.

For purposes of illustration, th reading light is shown as a line beam36 having no thickness, the beam 36, within the crystal 10, being at anangle B to the normal to the crystal surface 14. Also, the crystal i0 isshown containing alternating information bit regions A and 8 eachcontaining uniformly polarized domains, the polarization state of thedomains of the regions A being the opposite of the polarization state ofthe domains of the regions B. Also, for purposes of illustration, theeffects of field fringing are ignored, the various regions having theminimum width possible for the size of the writing beam used, and theregions being shown contiguous to one another for miximum informationstorage. Finally, as previously noted, the axis of each information bitregion A and B is perpendicular to the crystal surfaces.

Thus, in the instant illustrated case, owing to the high density of thestored information, the angle of the reading beam 36, and the size ofthe various regions, the reading beam 36, which is not parallel to theaxes of the regions A and 8, passes through two adjoining regions A andB. Since each region corresponds to one bit of information, theinformation stored in either the A or B regions, or both, cannot bedetected.

To avoid this situation, the various regions A and B must be at least aswide (or spaced from one another a distance at least as great) as thelateral distance R traversed by the reading beam during its passagethrough the crystal. This is so in order that the reading light can passthrough the crystal without intercepting two different information bitregions. This distance R, which is a limitation on the opticalresolution of the prior art systems, is given, approximately, for smallangles ofB, by the equation:

where: 7 is the thickness of the crystal l0, and B is the angle ofinclination (in radians), within the crystal l0, ofthe reading beam asrelative to the c axis of the crystal.

In one prior art system, for example, the thickness 'l' of the crystalE0, of bismuth titanate, is l()() microns, and ,B is approximately 0.067radian. The resolution limit R of such system is thus, approximately,6.7 microns.

In accordance with the instant invention, the resolution limit of thesystem is significantly improved, as now described. v

With reference to NC]. 3, an improved device at) is shown which issubstantially identical to the prior art device shown in NOS. 1 and 2,with the exception of the presence of a second light transmittingphotoconductive layer 42 disposed on the surface 14 of the crystal It)beneath the electrode 20.

In one utilization of the device d0, information is written into thedevice using a writing light, a pencil beam 46, in the instantembodiment, which is perpendicular to the crystal surfaces 12 and 14. Inthe write-in process, the perpendicular light beam 46 is not refractedas it passes through the various materials of the device, and the beamof light passes entirely through the device in the normal direction.Where the light passes through the two photoconductive layers 16 and 42,low electric resistance paths are created, thereby giving rise to twoelemental electrodes 26 and 48 at the two surfaces 12 and M of thecrystal M), respectively.

The electric field between one pair of electrodes 26b and 38b isindicated by the use of dash lines 50 representing some of the electricfield lines. Owing to the fact that the field lines are forced toterminate at relatively small elemental electrodes, the field linescannot merely diverge continuously from one another, as in the prior artdevice shown in FIG. 1, but must reconverge at each electrode 26b and48b. Thus, in comparison with the electric field of the prior artdevice, the field lines 5t} follow more curved paths and increase inlength more rapidly with increasing lateral distance of the lines fromthe electrode 26b. That is, the strength of the electric field of thedevice shown in FIG. 3 falls off more rapidly with increasing lateraldistance from the elemental electrode 26b than does the electric fieldof the prior art device. Thus, with otherwise identical writing beams,applied voltage, and material thickness, the width of the informationbit regions formed in the device 30 is substantially smaller than thewidth of the information bit regions of the prior art device.

Experiments have shown, for example, that the difference in width of theinformation bit regions is at least as great as a factor of 5, and insome instances, as high as a factor of 10. The optical resolution of thedevice 40 is improved, in comparison with the prior art devices, indirect relation to the reduction in width of the information bitregions.

In another utilization of the device 40, as illustrated in FIG. 4, thewrite-in of information into the device is performed using light raysthat are directed through the crystal at an inclination identical to theinclination at which the reading light is directed through the crystal,the reading light inclination, as previously noted, being dependent onthe ferroelectric material being used.

Thus, as a result of the use of an inclined writing beam, the twoelemental electrodes 26c and 52 at the opposite surfaces 112 and 14,respectively, of the crystal which are produced by the writing beam arelaterally offset from one another. Some of the electric field lines ofthe electric field between two electrodes 26c and 52 at the right of thecrystal it) are represented by dash lines 54. As is the case with thedevice 40 shown in FIG. 3, the ends of the electric field lines areforced to terminate close to one another. Thus, the strength of theelectric field, as compared with the described prior art device, fallsoff rapidly with lateral distance from the electrode 26c. In the FIG. 4embodiment, as compared with the FIG. 3 embodiment, however, the axis ofthe electric field, indicated by the arrow 58, in FIG. 4, is inclined tothe crystal surfaces at an angle equal to the angle of inclination ofthe writing beam. As a result, the

axis of the information bit region 60, shown at the right of thecrystal, is not disposed normal to the crystal major surfaces, as in theprior art device shown in l-iGv l, but is at an inclination parallel tothe paths through the crystal of both the writing and reading raysoflight.

The effect of the inclination of the information bit re gions bi) isillustrated in FIG. 5. In the figure, a device 40 is shown containingalternate regions A and B of stored information, the different regionsbeing contiguous to one another for maximum information storage, and thepolarization state of the domains of the regions A being the opposite ofthe polarization state of the domains of the regions B. Also, as in thedescription of the prior art device, the effects offield fringing areignored.

The reading light, a light beam 56 having no thickness, for purposes ofillustration, passes through the crystal at some inclination determinedby the particular ferroelectric material being used. As previouslynoted, however, the axes of the various regions A and B are parallel tothe path of the reading light beam, whereby there is virtually nopossibility of the reading beam passing through two adjacent regions Aand B. Thus, no resolution limit, as in the prior art device, arises byvirtue of the need to use an inclined reading beam, and, ignoring theeffects of fringing fields, the regions A and B can be as narrow andclose together as possible for the writing beam used.

In another embodiment, not illustrated, the conductive electrode 2%) isnot transparent, but has a light re flective'surface facing the crystal10. The device can be operated as the device 30, as described above, butwith the provision that the writing beam is of such character, i.e.,with respect to intensity or wave length, as to be substantiallycompletely absorbed in a single passage through the two photoeonductivelayers 36 and 42. That is, the wriring beam is not reflected by thereflective surface.

The reading beam, however, is of such character as to be reflected backout of the device through the layers i6 and i3, and through a pair ofcrossed polarizers disposed at the surface 12 side of the crystal.

Preferably, the information bit regions written into this device are ofsuch width that the reading light passes through the same region twiceon its round trip through the crystal. An advantage of this arrangementis that the optical effect on the light is doubled, thereby facilitat ngthe detection process.

Another embodiment of the instant invention is shown in FIG. 6. In thisembodiment, the device 76 comprises a crystal it) having aphotoconductive layer 15 convered by an electrode layer 18 as in theother devices described above. Disposed in spaced relation along thelower surface 14 of the crystal it), however, are a plurality oftransparent elemental electrodes '72 of, for example tin oxide-Coveringthe electrodes '72, and the crystal surface 1 9 between the electrodes,is a transparent layer 74 of an insulating material preferably having alow dielectric constant and a high resistivity, for example, silicondioxide. Disposed on the layer 74 is a transparent electrode 76 of, forexample, tin ox ide, connected to each of the electrodes 72 by means oflinks 78 extending through openings through the layer 74. A voltage isapplied between each electrode 72 and the electrode ifl on the othersurface of the crystal it) by connecting a voltage source (not shown)between the electrodes It; and 76.

The device '70 has particular utility in applications where the patternof the writing light on the crystal it) is fixed, and the device 70 isused to record either the presence of or absence of portions of thelight pattern.

Thus, for example, as shown to the left side of the crystal 10, anelectrode '72 is disposed directly beneath an elemental electrode 26formed by a writing light beam 80. In this embodiment, the angle of thewriting beam relative to the crystal is immaterial. The electric fieldbetween the elemental electrode 26 and the electrode 72 is relativelyconfined therebetween, whereby fringing of the electric field isreduced.

Alternatively, as shown to the right side of the crystal 10, anelectrode 72 is disposed laterally offset from the elemental electrode26, whereby the main axis of the electric field therebetween is inclinedto the crystal surfaces. Preferably, the inclination of the electricfield axis is the same inclination as that of the light used to read outthe information stored in the device. Thus, the optical resolution ofthe device is not limited, as is the prior art devices, owing to theneed for using inclined reading light.

It is noted that, in this embodiment, the inclination of the electricfield is obtained by virtue of the offset relationship between theelectrode 72 and electrode 26. Thus, the angle of the writing beam 80 isimmaterial.

in one embodiment, the electrodes 72 have a thicl ness of about 300 A,and a diameter of 1 mil. The electrode layer 76 has a thickness of about300 A, and the insulating layer '74 a thickness of about 1,000 A.

The various other layers of the devices shown, to the extent thatsimilar layers are used in the prior art devices, can have the samedimensions of the prior art similar layers. By way of example, theelectrode layers 18 and 20 can have a thickness of about 300 A, and thephotoconductive layers 36 and 2.2 can have a thickness of about 10,000A.

While the invention has been described in connection with ferroelectricmaterials, i.e., crystalline materials having domains which can bepolarized to stable states, thus providing memory action, the inventioncan be used with materials not considered classical ferroelectricmaterials in that the polarized states are not stable, i.e., thepolarized domains relax or return to the unpolarized state upon removalof the polarizing force. However, in that the the relaxing phenomenonrequires some finite time, devices according to this invention usingsuch materials have utility in systems requiring only short memories,e.g., buffer memories in computers, or display devices haing short frametime memory, e.g., television type displays. Examples of such materialsare various isomorphs of the material potassium-dihydrogen-phosphate (KHPOQ. These materials, it is noted, are ferroelectric materials attemperatures below the Curie points thereof.

Also, the instant invention has utility with devices using opticallyaddressed magneto-optic materials, such as yttrium or tcrbium irongarnet.

We claim:

l. An optical storage device comprising: an electrooptical body having apair of surfaces related to the crystallographical orientation of saidbody such that domains within said body are polarizable between twostates in rsponse to the application of an electric field through saidbody via said surfaces,

the polarized state of said domains being determinable by shining lightthrough one ofsaid surfaces at a preselected ahgle other than the normalthereto,

means, responsive to the illumination of said device, for providing anelemental electrode at each of said surfaces, said elemental electrodesproviding the sole means whereby an electric field can be appliedthrough said body,

said elemental electrodes being so disposed relative to one anotherwhereby the main axis of an electric field between said electrodes isdisposed at said preselected angle with respect to said surface andwherein each of said surfaces has a light transmit ting photoconductivematerial layer thereon, and each of said photoconductive layers has atranspan ent electrically conductive layer on the outer surface thereof.

2. An optical storage device comprising:

an clectro-optical body having a pair of surfaces related to thecrystallographical orientation of said body such that domains withinsaid body are polarizable between two states in response to theapplication of an electric field through said body via said surfaces,

means, responsive to the directing of a beam of light through said bodyand through both saidsurfaces, for providing an elemental electrode ateach of said surfaces, the positions of said electrodes at each of saidsurfaces being determined independently of one another by the surfaceintercepts of said beam, said elemental electrodes being the sole meanswhereby an electric field can be applied through said body and whereineach of said surfaces has a light transmitting photoconductive materiallayer thereon, and each of said photoconductive layers has a transparentelectrically conductive layer on the outer surface thereof.

3. An optical storage device comprising:

a flat member of an electrooptical material,

a light transmitting photoconductive layer on each of the major surfacesthereof,

a conductive layer on each of said photoconductive layers, at least oneof said conductive layers being transparent, and

means for directing light onto said member through said transparentconductive layer for writing infor mation into said device and fordetecting the information written therein.

4. An optical storage device comprising:

a flat member of an electro-optical material,

a light transmitting photoconductive layer on each'of the major surfacesthereof, and

a conductive layer on each of said photoconductive layers, at least oneof said conductive layers being transparent.

5. A device as in claim 4 wherein both of said conductive layers aretransparent.

6. A device as in claim 4 wherein the surface facing said member of theother of said conductive layers is light reflecting.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 r r075 Dated July 7! 9 3 lnventofls) Scott Allen Keneman and George W.Taylor It is certified that error appears in the above-identified patentand that said Letters Patent are hereby corrected as shown below:

Column 2, line 19., change "between" to --the particular state dependingupon--'. Column 8, line 43, after on, insert --the outersurface of--;and line 54, after on, insert ---the outer surface of--. 1

Signed and sealed this 6th day of August 197A.

(SEAL) Attest:

MCCOY M. GIBSON, JR. C. MARSHALL DANN Attesting Officer Commissioner ofPatents

2. An optical storage device comprising: an electro-optical body havinga pair of surfaces related to the crystallographical orientation of saidbody such that domains within said body are polarizable between twostates in response to the application of an electric field through saidbody via said surfaces, means, responsive to the directing of a beam oflight through said body and through both said surfaces, for providing anelemental electrode at each of said surfaces, the positions of saidelectrodes at each of said surfaces being determined independently ofone another by the surface intercepts of said beam, said elementalelectrodes being the sole means whereby an electric field can be appliedthrough said body and wherein each of said surfaces has a lighttransmitting photoconductive material layer thereon, and each of saidphotoconductive layers has a transparent electrically conductive layeron the outer surface thereof.
 3. An optical storage device comprising: aflat member of an electro-optical material, a light transmittingphotoconductive layeR on each of the major surfaces thereof, aconductive layer on each of said photoconductive layers, at least one ofsaid conductive layers being transparent, and means for directing lightonto said member through said transparent conductive layer for writinginformation into said device and for detecting the information writtentherein.
 4. An optical storage device comprising: a flat member of anelectro-optical material, a light transmitting photoconductive layer oneach of the major surfaces thereof, and a conductive layer on each ofsaid photoconductive layers, at least one of said conductive layersbeing transparent.
 5. A device as in claim 4 wherein both of saidconductive layers are transparent.
 6. A device as in claim 4 wherein thesurface facing said member of the other of said conductive layers islight reflecting.